1. Field of the Invention
The present invention relates to a shape of a magnetoresistive element, and is used for, in particular, a magnetic random access memory.
2. Description of the Related Art
In recent years, with development of a magnetic memory element indicating a giant magneto resistive (GMR) effect, an element having a ferromagnetic tunnel junction has been used as a memory element of a magnetic memory.
The ferromagnetic tunnel junction is comprised a stacked structure made of a ferromagnetic layer, an insulating layer, and a ferromagnetic layer, and a tunnel current flows in the insulating layer by applying a voltage between the two ferromagnetic layers. In this case, a junction resistance value changes in proportion to a cosine of a relative angle of a magnetization orientation of the two ferromagnetic layers.
Therefore, the junction resistance value is obtained as the smallest value when the magnetization orientations of the two ferromagnetic layers are identical to each other (in a parallel state), and conversely, the junction resistance value is obtained as the largest value when the magnetization orientations of the two ferromagnetic layers are opposite to each other (in an anti-parallel state).
A phenomenon that such a junction resistance value changes depending on magnetization patterns of the two ferromagnetic layers is called a tunneling magneto resistive (TMR) effect. Recently, there has been reported that a change rate (MR ratio) of a resistance value of a magneto tunnel junction (MTJ) element caused by the TMR effect becomes 49.7% at a normal temperature.
In the magnetoresistive element having the ferromagnetic tunnel junction, one of the two ferromagnetic layers is defined as a pinned layer having a magnetization pattern fixed thereto, and the other is defined as a free layer in which a magnetization pattern changes according to data. In addition, when magnetization of the pinned layer and the free layer is in a parallel state, it is defined as “0”, and when the magnetization is in an anti-parallel state, it is defined as “1”.
A data write operation is carried out by providing a magnetic field generated by a write current supplied to a write line to a magnetoresistive element, and inverting the magnetization orientation of the free layer of the magnetoresistive element. A data readout operation is carried out by supplying a readout current to the ferromagnetic tunnel junction of the magnetoresistive element, and detecting a resistance change of the ferromagnetic tunnel junction caused by the TMR effect.
A magnetic memory is configured by disposing such magnetoresistive elements in an arrayed shape. With respect to an actual configuration, one switching transistor is connected to one magnetoresistive element as in, for example, a dynamic random access memory (DRAM) such that the magnetoresistive element can be accessed in random.
Further, there has been proposed a technique of disposing a magnetoresistive element in which a diode and a ferromagnetic tunnel junction are combined with each other at a position at which a word line and a bit line cross each other.
Considering high integration of the magnetoresistive element having the ferromagnetic tunnel junction, a cell size must be reduced, and thus, the size of the ferromagnetic layer of the magnetoresistive element becomes inevitably small.
Here, as the property of the ferromagnetic layer, a magnetic structure (magnetization pattern) of the ferromagnetic layer is comprised a plurality of magnetization zones. In the case of a rectangular ferromagnetic layer, the magnetic structure of a center portion in a long axis direction configures a magnetization zone in which magnetization is oriented in a direction along a long edge, while the magnetic structure of both ends in the long axis direction configures a magnetization zone in which magnetization is oriented in a direction along a short edge, a so-called edge domain.
The edge domain causes reduction of an MR ratio caused by the TMR effect, and a rate of the reduction of the MR ratio caused by the edge domain becomes larger as the size of the ferromagnetic layer becomes smaller. In addition, when switching (magnetization inversion) of a magnetization pattern of the ferromagnetic layer is carried out, a change of the magnetic structure becomes complicated. This not only causes occurrence of noise, but also makes a coercive force large, so that a switching magnetic field increases.
In order to solve this problem, a magnetoresistive element has been proposed in which the shape of a free layer (ferromagnetic layer) becomes asymmetrical to a easy axis, for example, a parallelogram (refer to Jpn. Pat. Appln. KOKAI Publication No. 11-273337, for example).
According to this technique, the edge domain is small, and thus, a single magnetization zone can be configured over the substantially whole free layer.
On the other hand, a technique of applying a hard bias to an end of a free layer (ferromagnetic layer) to always fix an edge domain has been proposed as a method of preventing a complicated change of a magnetic structure of a ferromagnetic layer at the time of switching (refer to U.S. Pat. No. 5,748,524, for example).
In addition, there has been proposed a technique of newly adding a portion which protrudes in a direction vertical to an easy axis direction to a rectangular free layer (ferromagnetic layer) to form the shape of the free layer as an H shape or an I shape (refer to U.S. Pat. No. 6,205,053, for example).
In this manner, the shape of the free layer is formed as the H shape or the I shape, whereby it becomes possible to prevent a complicated change of the magnetic structure of the ferromagnetic layer at the time of switching and to reduce a switching magnetic field.
In the meantime, when the size of the ferromagnetic layer becomes small, its coercive force becomes large. The size of the coercive force becomes a milestone of the size of the switching magnetic field required for inverting magnetization. Thus, increasing the coercive force denotes increasing the switching magnetic field of the magnetoresistive element.
Therefore, when the size of the ferromagnetic layer becomes small due to downsizing of the magnetoresistive element, there is a need for a large write current at the time of writing data, which brings an unfavorable result such as increased power consumption or shorter service life of wiring.
From this fact, it is an indispensable object to achieve downsizing of the magnetoresistive element and reduction of the coercive force of the ferromagnetic layer used therefor at the same time in order to practically use a magnetic memory having a large capacity.
In order to solve this problem, a magnetoresistive element in which a free layer is comprised at least two ferromagnetic layers and a nonmagnetic layer disposed therebetween has been proposed (refer to U.S. Pat. No. 5,953,248, for example).
In this case, the two ferromagnetic layers are different from each other in magnetic moment or thickness, and are opposed to each other in magnetization orientation due to anti-ferromagnetic junction. Thus, since the influence due to magnetization is effectively offset from each other, it can be considered that the whole free layer is similar to a ferromagnetic body having small magnetization in the easy axis direction.
When a magnetic field is applied to an orientation opposite to that of small magnetization of the easy axis direction, magnetization of the ferromagnetic layer is inverted while maintaining the anti-ferromagnetic junction. At this time, since a magnetic force line is closed, the influence due to the anti-magnetic field becomes small. In addition, because the switching magnetic field of the free layer depends on the coercive force of the ferromagnetic layer, it becomes possible to invert magnetization in a small switching magnetic field.
In the case where an inter-layered junction is not present between the two ferromagnetic layers, interaction caused by a static magnetic junction occurs due to a leakage magnetic field from these ferromagnetic layers. In this case as well, the switching magnetic field can be reduced (refer to transaction of the 24th Japan Applied Magnetic Society, 12aB-3, 12aB-7, transaction of the 24th Japan Applied Magnetic Society, pp. 26 to 27, for example).
However, in the case where no inter-layered junction is present between the two ferromagnetic layers and only a static magnetic junction exists, the magnetic structure of the ferromagnetic layers becomes unstable. In this case, a rectangular shaped ratio in a hysteresis curve or magnetic resistive curve becomes small, and it becomes difficult to obtain a large magnetic resistive ratio. Thus, this magnetoresistive element is not preferable.
As described above, it becomes an indispensable element to avoid complication and to ensure stability with respect to the magnetization zone of the free layer in order to obtain a large output signal with less noise.
However, in general, a parallelogram-shaped free layer is simple in magnetic structure and is obtained as a substantially single magnetic zone, while the coercive force and switching magnetic field become large.
In addition, the behavior during magnetization inversion can be controlled by adding the hard bias structure for fixing an edge domain to the end of the free layer, and in this case also, the coercive force increases. In addition, since there is a need for addition of the hard bias structure for fixing the edge domain, this structure is not suitable to higher density required for a large capacity memory or the like.
Further, in the H-shaped or I-shaped free layer, there is a need for increasing the portion which protrudes in the direction vertical to the easy axis direction in order to introduce to the maximum an advantageous effect of preventing a complicated change of the magnetic structure of the ferromagnetic layer at the time of switching. In this case, however, the size of the magnetoresistive element substantially increases, and therefore, this structure is not suitable to high integration required for a large capacity memory or the like.
It is an indispensable element to reduce a switching magnetic field in order to achieve a magnetic memory, for example, a magnetic random access memory. However, if the free layer is downsized, for example, if the width of the free layer in the short axis direction becomes in order from several microns to sub-microns, a magnetic structure (edge domain) which is different from a magnetic structure of a magnetic object at the center portion is generated due to the influence of an anti-magnetic field at the end of the free layer (magnetization region).
A switching curve of the MTJ element having the magnetic tunnel junction is important during a data write operation in a magnetic random access memory.
The MTJ element is disposed at a cross portion of two write lines which cross each other. A data write operation is carried out by inverting the magnetization orientation of the MTJ element due to the magnetic field generated by a current supplied to the two write lines. A data write operation for the MTJ element is not carried out in only the magnetic field generated by a current supplied to one of the two write lines.
Therefore, the switching curve is defined by the size of a magnetic field in an easy axis direction and the size of a magnetic field in a hard axis direction which are required for switching (magnetization inversion), on a plane formed by the easy axis and the hard axis of the free layer of the MTJ element.
The switching curve is known as being expressed as an asteroid curve in a single magnetic zone model. The write characteristic substantially depends on the switching curve. Thus, an attempt has been made for deforming the switching curve to significantly obtain a write window or increasing stability of the MTJ element in a half selective state in which only a magnetic field generated by a current flowing to one of the two write lines is applied.
Then, a proposal of deforming the shape of the MTJ element has been made as a method of achieving such a switching curve.
For example, a broad bean type (C-type) MTJ element is provided (refer to Jpn. Pat. Appln. KOKAI Publication No. 2003-78112, for example). The broad bean type MTJ element is featured in that the magnetic structure (magnetization pattern) configures a C-type magnetic zone when the magnetic field in the easy axis direction is small, and that the magnetic structure configures an S-type magnetic zone when the magnetic field in the hard axis is large.
When the magnetic structure configures the C-type magnetic zone, the magnetization orientation of the free layer is hardly inverted, and when the magnetic structure configures the S-type magnetic zone, the magnetization orientation of the free layer is easily inverted. This makes it possible to prevent incorrect writing relevant to the MTJ element in a half selective state, and to lower the coercive force at the time of writing to reduce the switching magnetic field.
In addition to the broad bean type, a cross shape exists as a shape such that the magnetic structure when the magnetic field in the hard magnetic axis direction configures the C-shape magnetic zone. The cross-shaped MTJ element is featured in that the magnetic structure when the magnetic field in the hard axis direction is small configures two C-shape magnetic zone. In the cross-shaped MTJ element, a switching magnetic field in a direction forming 45 degrees with respect to the easy axis or hard axis can be reduced.
However, in any case, the existing MTJ element having a broad bean shape or a cross shape cannot attain a write characteristic which can fully satisfy the requirements. Further, there is a demand for the shape capable of providing a wide write window and stabilizing the state of the MTJ element in a half selective state.